The joining of articles by filling the joint between the two articles with a low melting point material is known conventionally as brazing. Conventionally, brazing filler metals have a low melting point in relation to the articles being joined, sufficient mechanical strength when solid to create an effective joint and the ability to wet the surface of the articles being joined when the brazing filler metal is in the molten state. While pure metals and eutectic compositions having a single temperature melting point and are used for brazing, it is most common to use alloys of metals having a liquidus and solidus, rather than a single temperature melting point. In connection with the present invention, the term melting point will be used in a non-eutectic alloy to mean its solidus and its flow temperature will be used to indicate the liquidus. The term brazing filler metal is used herein to refer to the metal that is melted to form the braze joint, whether it be a pure metal or an alloy of two or more elements. The term brazing alloy is used herein to refer to a brazing filler metal that is a metallic alloy.
Conventional brazing techniques are used in a wide variety of applications. They may join dissimilar metals, similar metals, nonmetals such as graphite, metal matrix composites, metal coated articles such as an electroplated ceramic. In all these applications, some means of placing the brazing filler metal in the proper location, configuration and amount, compatible with the anticipated joint structure must be used. Conventionally, brazing filler metals are available as rods, ribbon, powder, paste, cream, wire, sheet, strip and foil. These forms of brazing filler metals may be further shaped to specific configurations specifically adapted to the configuration of the joint to be formed. The brazing filler metal so shaped is then place between the articles to be joined, the temperature raised to the flow temperature of the brazing filler metal and then reduced below the melting point to form the joint.
It is also known to electroplate at least one component of the brazing filler metal on portions of an assembly to be joined, with the material electroplated forming the brazing filler metal. In addition, recently there are available what are known as transfer tapes where the brazing filler metal and an adhesive are used to form a brazing filler metal configuration compatible with the configuration of the joint to be formed.
The major drawback with all of these forms of brazing filler metals is the relative difficulty in forming the brazing filler metal to the correct configuration and then retaining that shaped brazing filler metal in the proper location with respect to the anticipated joint through the brazing operation. While electroplating solves several of those shortcomings, it has its own unique shortcomings. For example, there exist restrictions on the compositions available for electroplating as it is difficult to electroplate certain alloy compositions that may be desired for a brazing filler metal. In addition, there is the requirement that the portions of a device not receiving the plating must be masked to prevent the entire article from receiving the electroplated brazing material.
Therefore, it is one object of the present to provide a method of brazing articles where the brazing filler metal is readily located only on the portions of the device forming the brazed joint. An additional object of the invention is to locate the brazing filler metal on the portion of the article to be joined without the necessity for complex fixtures or other means of physically retaining a separate article comprised of the brazing filler metal. Still a further object of the invention is to provide a method of brazing articles adapted to automation.
Still further objects of the invention will be apparent to those skilled in the art, especially in light of the present specification.